The expansion vessel is not knackered. Period.
And as for servicing unvented cylinders?
Rarely happens in the real world.
14 years you said, and not been ever touched you said? 1.5 bar pressure still in the air pressure??
I doubt that could be correct.
I wish they would keep some air pressure.
Either you got the reading wrong by taking the pressure by wrong method, or someone has actually been doing the air charge.
Note that if the air gap volume is too small, the pressure can still be reading sort of decent. It requires volume of air as well as pressure. Hence why the supplied vessels have greater volume on larger unvented cylinders.
For all you know, the 14 year old prv could be faulty and the unit and vessel could have 5 bar mains water on it.
The diaphragm (often balloon membrane) will absorb most of the air pressure every year. The balloon will expand as the air side decreases. It will also stick to the outer casing. The potable vessel is normally NOT stainless steel, but mild steel.
The vessel will be knackered, trust me.
It isn’t designed and built to last that length of time and I certainly wouldn’t risk leaving it as mild steel vessels will pin hole and flood the property with high pressure mains.
The membrane is flexing constantly when heat/pressure rises and drops and when a tap is opened and closed and only has a limited life.
The expansion vessel is a basic serviceable part and because it hasn’t been ever serviced and is 14 years old means a G3 plumber will replace it if they know that.
As for servicing unvented cylinders, - you are correct, most are neglected, but that doesn’t mean it is safe. And surely this thread is about doing an illegal diy part service anyhow, which you obviously think is necessary?
You are best to just do the unvented qualification, as it is little cost.
